Preparation of magnetic coating

ABSTRACT

A magnetic coating composition is disclosed of the type having magnetic particles dispersed in a resin binder. The improvement comprises the use of a solvent system including a dibasic ester wherein the solvent system has an initial Hansen three-dimensional solubility parameter range of approximately 10.5 to 11.0 and a Hansen three-dimensional solubility parameter range of approximately 9.3 to 10.0 after approximately 99% of the solvent has been evaporated from the binder.

This application is a continuation of patent application Ser. No.495,554, filed May 17, 1983 now abandoned.

BACKGROUND OF THE INVENTION

Prior art magnetic coating compositions have been fabricated using boththermoplastic and thermosetting binder systems. Although many of theprior art binder systems exhibit those desirable characteristics fortheir particular intended uses, most prior art systems exhibit poordurability as measured by either head crash resistance or their abilityto withstand physical abrasion. Further, most prior art binder systemsexhibit cratering, pinholes, discontinuities in the coating, low signalamplitude, bit-shift, poor overwrite, low resolution, poor signal tonoise ratio, poor surface finish, high switching field distribution, loworientation ratios, as well as low magnetic squareness and poordispersion quality.

Generally, in order to improve one or more of the physical, electricalor magnetic characteristics of the coating, it has been necessary tocompromise certain characteristics, the choice being dependent upon thefinal end use of the coating.

There are many reasons for the above mentioned deficiencies in priorbinder compositions. One of the most important is poor dispersion of themagnetic oxide in the coating. For example, the phosphate esterdescribed in this invention produces improved dispersions when comparedto other prior art surfactants. Thus, improved magnetic oxide dispersionresults in enhanced coating continuity and surface smoothness, which inturn results in an increase in signal amplitude, improved signal tonoise ratio and superior magnetic parameters including orientationratio, switching field distribution and squareness. It was also learnedthat excessive bit-shift is due to poor dispersion. In addition, mostprior art binder systems incorporated a catalyst system based on anamine which cures through the epoxy groups. Solid expoxy resins claimedin prior art have a high epoxy equivalent which limits the amount ofcross linking that can occur. If, however, a Lewis-Acid type catalystsuch as an acidic phosphate ester is used, cross linking occurs betweenthe epoxy and phenolic resins which leads to a three dimensional networkwith improved durability.

It is thus an object of the present invention to provide a magneticcoating composition without the above-recited deficiencies of prior artcompositions.

It is yet another object of the present invention to provide a magneticcoating composition which exhibits enhanced dispersion qualitycharacteristics over those compositions of the prior art.

It is yet another object of the present invention to provide a magneticcoating composition exhibiting greater durability, fewerdiscontinuities, greater signal amplitude as well as improved bit-shift,overwrite, resolution, signal to noise, switching field distribution andmagnetic squareness characteristics.

It is still another object of the present invention to provide amagnetic coating composition with an improved dispersant to performthose objects as recited above.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is a magnetic coating composition of the typehaving magnetic particles dispersed in a resin binder. The improvementcomprises the use of a solvent system including a dibasic ester whereinsaid solvent system has an initial Hansen three-dimensional solubilityparameter range of approximately 10.5 to 11.0 and a Hansenthree-dimensional solubility parameter of approximately 9.3 to 10.0after approximately 99% of the solvent has evaporated from the binder.

It has been determined that a magnetic coating composition having theabove-recited characteristics can be achieved where said compositioncontains a phosphate-ester dispersant and a cellosolve/dibasic estersolvent. Further solvents which are capable of use with thephosphate-ester dispersant in yielding the above-recited characteristicsare isophorone/dibasic ester and cyclohexanone/dibasic ester.

DETAILED DESCRIPTION OF THE INVENTION

The objects of the present invention are first achieved through the useof a phosphate ester dispersant, which functions in a dual role.Firstly, the preferred binder composition of the present invention isthe result of an epoxy-phenolic reaction and the phosphate estercatalyzes such a reaction. The phosphate ester also acts as a dispersantfor the magnetic oxide pigment.

The solvent system is also unique. The system allows one to maintain aclose control over resin compatibility during coating, orientation anddrying. The end result is an extremely smooth continuous coating.

The present invention is not in the resin system, but in the novelcombination of a phosphate ester dispersant and specific solvent havingpreviously defined Hansen three-dimensional solubility parameter ranges.However, for the sake of illustration, the following represents atypical coating composition for use in practicing the present invention.

The preparation of the coating involves three separate operations,namely, ball milling, letdown and filtration. The milling step wascarried out in a ceramic-lined ball mill loaded to 50% volume with 3/8inch diameter zirconia cylinders. The milling solution comprised thefollowing:

    ______________________________________                                        Mill Base          Proportions                                                                             Solids                                           ______________________________________                                        Magnetic Oxide     34.38     34.38                                            60% Weight Epon 1004                                                                              3.27      1.96                                            Cellosolve         41.94     --                                               Dibasic Ester      18.62     --                                               Gafac RE-610        1.79      1.79                                                               100.00    38.13                                            ______________________________________                                    

Gafac RE-610 is a reaction product of mono and dibasic esters oforthophosphoric acid and a nonionic surfactant, also known as Igepalavailable from GAF Corporation. Epon 1004 is a condensation polymer ofepichlorohydrin and bisphenol A available from Shell Chemical Co.

The milling solution was charged to the ball mill by adding first thedibasic ester solvent, followed by the 60% Epon 1004 solution incellosolve, which is monoethyl ether of ethylene glycol. This wasfollowed by the Gafac RE-610 and magnetic iron oxide.

The milling solution was milled for approximately 120 hours at 35°-40°C. in order to achieve the required degree of dispersion.

A letdown composition was then prepared containing the followingcomponents:

    ______________________________________                                        Mill Base        Proportions                                                                             Solids                                             ______________________________________                                        60% Epon 1004    16.51     9.91                                               Methylon 75108   5.95      5.95                                               10% Butvar B-74  7.43       .07                                               Cellosolve       8.00      --                                                 Dibasic Ester    15.58     --                                                 Dow-Corning Paint                                                                              0.65       .07                                               Additive No. 11                                                               Alumina          1.40      1.04                                               ______________________________________                                    

Methylon resin 75108 is a phenolic resin available from the GeneralElectric Company. Dow-Corning Paint Additive No. 11 is a silicone resin.

The letdown solution was prepared by combining dibasic ester solvent,60% Epon 1004 in cellosolve, methylon resin 75108, 10% polyvinyl butyralavailable from Monsanto as B-74 and cellosolve. The resulting solutionwas stirred for one hour and filtered through an 8 micron absolutefilter, the filtrate then slowly added to the mill base over a period of45-60 minutes with moderate stirring. Finally, the Dow-Corning Paintadditive and dispersed alumina in Epon 1004 was added while stirringover a period of 10 minutes.

At this point, the coating was complete except for filtration andviscosity stabilization. The coating was stirred at a high shear for an18-hour period prior to filtration through a 1 micron nominal and 8micron absolute filters. After filtration, the coating was allowed tostabilize by stirring or paddling under low shear for at least 24 hoursprior to use.

The dibasic esters useful in practicing the present invention areavailable from E. I. du Pont de Nemours & Co. as mixtures and separatecomponents of three esters, namely, dimethyl succinate, dimethylglutarate and dimethyl adipate. They possess the general chemicalformula of H₃ COOC(CH₂) X COOCH₃, where X is an integer from 2 to 4.

Although historically the magnetic disc industry has relied upon epoxyand phenolic resins for their binder systems, there are other resinsystems which are possible. The thrust of the present application is theuse of a phosphate-ester dispersant and a solvent system having aninitial Hansen three-dimensional solubility parameter range ofapproximately 10.5 to 11.0 and a Hansen three-dimensional solubilityparameter range of approximately 9.3 to 10.0 after approximately 99% ofthe solvent has evaporated from the binder. Preferably, the presentinvention employs a phosphate-ester dispersant in a cellosolve/dibasicester solvent to achieve the improved physical properties recited above.

While certain specific embodiments of this invention have beenillustrated and described in detail herein, it is obvious that manymodifications thereof may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of preparing a phenol-epoxy magneticdisk coating recording composition including magnetic particlesdispersed in a phenolic-epoxy resin binder, the method including thesteps of: intermixing a solvent with sufficient of a phosphate-ester toboth disperse the particles adequately as well as to catalyze sufficientcross-linking of phenolic and epoxy resin to yield a three-dimensionalnetwork of enhanced durability; the solvent including a dibasic estercomponent.
 2. The method as recited in claim 1 wherein the solventsystem is selected and adapted to exhibit a Hansen three-dimensionalsolubility above of approximately 10.5, initially, and a Hansenthree-dimensional solubility below approximately 10.5 after drying. 3.The method of claim 2 wherein said composition is formulated to containan acidic phosphate-ester dispersant/catalyst and wherein the dibasicester solvent also includes cellosolve.
 4. The method of claim 3 whereinthe phosphate-ester dispersant/catalyst and cellosolve-dibasic estersolvent are selected and arranged to comprise, together, about 5% byweight of the total binder composition.
 5. The method of claim 3 whereinthe solvent is selected and arranged to exhibit a Hansenthree-dimensional solubility of about 10.5-11.0 initially and about 9.3to 10.0 after approximately 99% of the solvent has evaporated from thebinder.
 6. The product of the method in claim
 3. 7. The method asrecited in claim 3 wherein the phosphate ester is selected as thereaction product of mono and dibasic esters of a phosphoric acid and anon-ionic surfactant.
 8. The method as recited in claim 7 wherein thephosphate-ester dispersant/catalyst and the cellosolve-dibasic estersolvent are selected and arranged to exhibit a Hansen three-dimensionalsolubility of about 10.5-11.0 initially and about 9.3 to 10.0 afterapproximately 99% of the solvent has evaporated from the binder.
 9. Thecombination as recited in claim 7 wherein the dibasic ester component isselected to comprise mixtures and separate components of a di-methylsuccinate, a di-methyl glutarate and a di-methyl adipate.
 10. Theproduct of the method in claim
 2. 11. The method as recited in claim 1wherein the dibasic ester component is selected to comprise mixtures andseparate components of a di-alkyl succinate, a di-alkyl glutarate and adi-alkyl adipate.
 12. The product of the method in claim
 11. 13. Theproduct of the method in claim
 1. 14. A method for formulating amagnetic disk recording coating which has magnetic recording particlesdispersed in a phenolic-epoxy resin binder, this methodcomprising:intermixing with a solvent system, a moiety of acidicphosphate ester, sufficient to disperse the particles adequately andalso sufficient to so catalyze the cross-linking of phenolic and resingroups as to create a high durability network; the solvent system beingselected to include a dibasic ester component.
 15. The product of themethod in claim
 14. 16. A method of preparing a magnetic recordingcoating composition of the type having magnetic recording particlesdispersed in a phenolic-epoxy resin binder, this method comprisingmixing a phosphate ester dispersant with a solvent system including adibasic ester, this solvent system being selected and adapted to exhibitan initial Hansen three-dimensional solubility of approximately 10.5 to11.0 and a Hansen three-dimensional solubility of approximately 9.3 to10.0 after approximately 99% of the solvent has evaporated from thebinder.
 17. The product of the method in claim
 16. 18. A method forformulating a magnetic disk recording coating which has magneticrecording particles dispersed in a phenolic-epoxy resin binder, thismethod comprising:intermixing with a solvent system, a moiety of acidicphosphate ester, sufficient to disperse the particles adequately andalso sufficient to so catalyze the cross-linking of phenolic and resingroups as to create a high durability network.
 19. The product of themethod in claim
 18. 20. A method of preparing a magnetic recordingcoating composition of the type having particles adapted to high speedmagnetic recording dispersed in a phenolic-epoxy resin binder, thismethod comprising mixing a phosphate ester dispersant with a solventsystem, and providing sufficient of the phosphate ester dispersant toboth disperse the particles adequately as well as to catalyze sufficientcross-linking of phenolic and epoxy resin to yield a three-dimensionalnetwork of enhanced durability.
 21. The method of claim 20 wherein saidcomposition solvent contains cellosolve.
 22. The method of claim 21wherein the phosphate-ester dispersant and cellosolve-dibasic estersolvent comprise about 5% of the total binder composition.
 23. Theproduct of the method in claim
 22. 24. The product of the method inclaim
 21. 25. The method of claim 20 wherein the dispersant and solventcomprise about 5% of the total binder composition.
 26. The product ofthe method in claim
 25. 27. The method of claim 20 wherein saidcomposition is formulated to contain an acidic phosphate-esterdispersant/catalyst and wherein the solvent comprises a dibasic ester.28. The product of the method in claim
 27. 29. The product of the methodin claim 20.